This invention relates generally to transmission of electrical power to sub-sea electrical equipment such as a motor driving a compressor/pump located far away from the shore, and more particularly to a system and method for sharing power among different source-side converter modules without a communication link in a transmission and distribution system that employs modular stacked DC (MSDC) technology for sub-sea applications.
Transmission of electrical power to oil and gas sub-sea electrical equipment often requires high power to be transmitted over long distances. Such a transmission is done at high voltages to reduce losses. At the receiving sub-sea end, the voltage is stepped-down and then distributed to the individual loads. Distribution distances are typically much shorter than the transmission distance.
Three phase 50/60 Hz AC power transmission and distribution is a mature technology. Using this technology, step-up transformers are used at the sending end to increase the voltage to transmission levels (e.g. 72 kV). At the subsea end, step down transformers are used to reduce the voltage to distribution levels.
AC transmission, although mature, provides technical challenges for applications where bulk power is transmitted over long cables. Due to cable capacitance, a significant amount of reactive power needs to be provided by the power source and carried by the cable. Capacitance causes charging current to flow along the length of the AC cable. Because the cable must carry this charging current in addition to the useful load current, the cable losses are high; the cables are over-rated and expensive. Large reactive power requirements may trigger power system stability issues. The limitation of 50/60 Hz AC transmission and distribution can be alleviated by reducing the power transmission frequency (e.g. 16⅔ Hz). This reduces the reactive power requirement by the cable capacitance. However, this solution is at the expense of increase in size of magnetic components such as transformers. At high power levels, the size and weight penalty would be excessive.
Typically, multiphase booster pumps require electrically driven motors delivering a shaft power between 2 MW and 6 MW. Such pump clusters may require power on the order of 20 MW to be transmitted over 50 kms.
Further, sub-sea motors driving a gas compressor typically have a higher nominal power (e. g., in the order of 10 or 15 MW). As such, sub-sea compression clusters may be required to transmit a total power in the order of 50 to 100 MW over a distance of 100 or 200 km. The transmission of high power over a distance of more than 100 km and distributing the power sub-sea is very challenging with AC transmission and distribution systems because of the high charging currents and the high number of components involved in the distribution system.
In general, DC transmission can be achieved more efficiently over long distances than AC transmission. High voltage (HV) DC transmission typically requires the usage of power electronic converters in the transmission systems that are capable of converting between HVAC and HVDC. Each switch of the converter for conventional HVDC converter topologies is designed to handle high voltages. The converter nominal voltage may range from tens-of-kilovolts to hundreds-of-kilovolts, depending upon the application. Such switches are typically configured utilizing a plurality of series connected semiconductor devices (e.g., such as insulated gate bipolar transistors (IGBTs) and thyristors). Because of the size and the high number of components involved, conventional HVDC terminals are not well suited for sub-sea installations.
Power converters are also required on the load side of a power distribution system. Typically, a power converter in conjunction with a high voltage transformer is used to step down the voltage from the DC transmission level to the voltage level used in the power distribution system. The variable speed drive (VSD) of the sub-sea pump/compressor motor converts this distribution level voltage to variable frequency AC voltage required to run the motor over a wide range of speed.
Modular stacked DC converter architectures are well suited for sub-sea applications requiring transmission and distribution over long distances. Unlike other DC transmission options, wherein the dc transmission (link) voltage is controlled, i.e. maintained nearly constant, the dc transmission (link) current is controlled in a modular stacked dc converter. One MSDC architecture 10 is depicted in FIG. 1. The MSDC architecture gets its name from the fact that the architecture uses several dc-dc/ac-dc/dc-ac converter modules stacked and connected in series on the dc side, both at the sending end and at the receiving end of the transmission link such as depicted in FIG. 1.
The sending end/top-side converters 12 comprise a set of ac-dc converters 14, which draw power from the ac mains or grid 16. Each of these converters 14 is cascaded with a dc-dc converter 18. These dc-dc converters 18 are connected in series and they are controlled so as to regulate the current in the dc cable 20 connecting the top-side 12 to the sub-sea installation 22. It shall be understood that the sending-end ac-dc 14 and dc-dc converter 18 stages (shown explicitly in FIG. 1) can be replaced by a single ac-dc converter that combines the functions of both the stages. The sub-sea/receiving-end 22 also comprises several dc-dc converters 19 connected in series. Each of these converters 19 is cascaded with a dc-ac inverter/motor drive 24. These dc-dc converters 19 are controlled to regulate the dc link voltage to that required by the down-stream motor drive 24. It shall also be understood that the subsea dc-dc 19 and motor drive 24 (shown explicitly in FIG. 1) can also be replaced by a single dc-ac converter that combines the functions of both the stages. Although FIG. 1 depicts two-level converters used for the ac-dc, dc-dc and motor drive modules, it shall be understood that at high power levels, multi-level stacks will be used for these converter modules.
The sending end converters 12 may derive power from a single or multiple sources/generators 30 located in one place as illustrated for one embodiment in FIG. 2. According to another embodiment, the sending end converters 12 may derive power from several sources 30 that are located in places separated by large geographical distances such as depicted in FIG. 3. In both of these embodiments, there is a need to share the total load power between the different connected sources 30. According to one embodiment, a supervisory controller can be employed that will communicate the power to be delivered by each module, wherein each module is connected to one source 30. This approach requires communication lines between the controller and each module. Any failure of a corresponding communication link or the supervisory controller itself will undesirably shut down the entire system. System reliability is one of the critical features in sub-sea transmission and distribution systems; and such single point failures are not acceptable.
In view of the foregoing, there is a need to share the total load in a sub-sea transmission and distribution system among the front-end converters equally or proportional to their ratings in a manner that is not susceptible to single point communication link failures such as those exhibited by systems that employ a supervisory controller.